Field of the Invention
The present invention relates to a coil unit including a coil and a ferrite having the coil arranged.
Description of the Background Art
Various contactless charging systems for transmitting electric power from an electric power transmission apparatus to an electric power reception apparatus in a contactless manner have conventionally been proposed (Japanese Patent Laying-Open No. 2013-154815, Japanese Patent Laying-Open No. 2013-146154, Japanese Patent Laying-Open No. 2013-146148, Japanese Patent Laying-Open No. 2013-110822, and Japanese Patent Laying-Open No. 2013-126327).
Japanese Patent Laying-Open No. 2008-120239 describes a coil unit including a coil and an E-shaped core. The E-shaped core is formed in a rectangular shape when viewed from above and includes projecting portions formed in two respective side portions and a central projecting portion formed in a central portion.
This E-shaped core is formed by stacking a plurality of block cores, and the block cores are arranged to be in contact with each other. The coil is attached to the central projecting portion of the E-shaped core.
In the coil unit described in Japanese Patent Laying-Open No. 2008-120239, however, the block cores are arranged without leaving any space therebetween. Therefore, a necessary amount of ferrite is large and manufacturing cost is high.
The inventors of the present application have studied about reduction in necessary amount of ferrite by forming a ferrite from a plurality of divided ferrites and arranging ferrites at a distance from each other.
A specifically studied ferrite includes a coil base in a form of a frame on which a coil is arranged and a central ferrite arranged to be in contact with an inner peripheral portion of the coil base and surrounded by the coil.
The inventors have formed the coil base by arranging divided ferrites in a square shape at a distance from each other in a form of a frame, and formed the central ferrite by arranging divided ferrites in a square shape at a distance from each other.
A divided ferrite located at a corner portion of the central ferrite and a divided ferrite located at a corner portion of the coil base are in point contact with each other, and an area of contact is small. Therefore, it has been found that magnetic saturation may occur in a portion of contact between the divided ferrites when a current flows through the coil or when an external magnetic flux intersects with the coil.
Then, the inventors have formed a coil base by radially dividing the coil base from the center of the coil base and arranging in a form of a frame, eight divided ferrites in a shape of a right-angled trapezoid. Specifically, the divided ferrites have been arranged such that a short side portion of each divided ferrite is disposed around an inner periphery of the coil base and an outer periphery of each divided ferrite is disposed around an outer periphery of the coil base. Thus, when a central ferrite is arranged on the coil base, each divided ferrite is in contact with the central ferrite at the short side portion of the right-angled trapezoid, and an area of contact between each divided ferrite and the central ferrite can be secured.
In application to a coil unit without a central ferrite as well, divided ferrites forming a coil base can be identical in shape.
The coil base above, however, suffers from a disadvantage that a large-sized coil base cannot be formed.
In general, a divided ferrite is formed by molding a powdery raw material for forming a ferrite in a mold die and firing a molded product. In manufacturing a large divided ferrite, a large molded product should be fired in firing. In firing, a temperature distribution tends to be uneven and a fracture or a crack tends to be produced. Therefore, it is currently difficult to manufacture a large divided ferrite.
Therefore, it has been found that the coil base studied by the inventors suffers from such a problem that only a small-sized coil base can be formed.
The present invention was made in view of the problems above, and an object of the present invention is to provide a coil unit including a coil base which can be applied also to a large-sized coil base and can ensure an area of contact with a central ferrite even when the central ferrite is arranged on an upper surface of the coil base.